When GSM (Global System for Mobile communications) was initially designed, it was designed as a Circuit Switched technology, to carry voice and signalling. As the industry developed there was a desire to transport data; GPRS (General Packet Radio Service) was thus designed. The design of GPRS was based upon GSM, however, in the end the two technologies were not wholly compatible, to the extent that only one technology could be active at any one time, with the voice element (GSM) generally taking precedence.
Some further design work was completed to allow the simultaneous use of packet switched data traffic and circuit switched voice, resulting in the technology called Dual Transfer Mode (DTM). Whilst DTM allows greater compatibility between GPRS and GSM, the costs for implementing it in handset and the network are significant, and it does not provide a decent data rate, instead only resulting in low packet data rates. Therefore most handsets and networks don't support the DTM functionality.
In contrast, the 3G technology (e.g. UMTS) from the beginning was designed with the particular aim to enable mobile terminals to access both the circuit switched (voice) and the packet switched (data) domains simultaneously, and therefore it became intrinsic to the technology.
Today, in many regions GSM networks coexist with 3G networks, and the majority of mobile terminals are capable of operating on both types of network.
To facilitate the coexistence of the GSM and 3G networks, handover functionality has been incorporated into their design, which allows an operator to provide seamless support for voice across the two technologies. The Handover facility was developed in order to allow terminals to be moved between the two technologies because of differences in coverage between the technologies, and therefore the Radio controller (BSC/RNC) is responsible for triggering the handover process.
However, since 3G has been deployed, there has been a big uptake in data traffic, resulting in greater congestion in the 3G networks. This congestion is compromising the ability to be able to provide the required peak data rates and performance.
There is therefore a need to overcome or at least ameliorate this problem.